January 11, 1771: The Birthday of Lake Alleghe

The lake of Alleghe in the valley of Cordévole
is today exactly 244 years old. The "birthday" of this lake is
well known, at 7:02 in the morning of January 11, 1771 the river flowing
through the valley became dammed by a landslide coming from the
mountain Piz.

Fig.1.
General view of the valley of Cordévole with the village and lake of
Alleghe. On the right, on the mountain Piz, the scar of the landslide is
barely visible in the forest, in the background the Civetta (3.220m).

The Alps-traveler Belsazar Hacquet (1739-1815) remembers a visit to the lake in 1780:

“The river Cordévole became my
guide, by following him I would find the valley of Cadore. But only
after some hundred steps the river was flowing in a large lake, existing
here only for the last nine years. I walked around in eastern
direction, to the villages of Sternade and Saviner until the mountain of
Piz. First the lake was narrow, only by Saviner it became more than 100
venetian fathom [an old length unit used in the mining industry of
these times, one fathom ca.1,8m] broad and more than thirty deep.The last mentioned village once was situated on a hill, before it in
a broad valley there were four smaller villages…[]…which became flooded
by the lake, but the fourth locality, named Marin, was buried with the
village of Riete under the collapse of the mountain of Piz, last
mentioned village situated previously on the top of the mountain.”Standing on the top of the mountain, I immediately noted that the
mountain has a volcano on top of it, and it was possible to see how deep
it went. After the mountain collapsed, it could be seen that its base
was composed of limestone, build up by mighty layers, dipping from the
west to the east with 45 degrees. The surface of the collapse is so smooth, that a man has difficulties to climb on it to the mountain.“

Fig.2.
Historic depiction of the landslide-lake in the “Atlas Tyrolensis” of
1774 by Tyrolean cartographers Peter Anich and Blasius Hueber. Note the landslide-boulders on the
southern shore of the lake, Anich and Hueber were one of the first
cartographers to use signatures to display geomorphologic features in
their maps (image in public domain).

The strange notion by Hacquet of an active volcano in the Dolomites
in very recent times is based maybe on his discovery of volcanic rocks in the area, however –
as we today know – these deposits are more than 235 million years old.
At the time of Hacquet’s geologic investigation volcanic forces were
also believed to cause strong and sudden movements of the terrestrial
surface, effects that maybe could also explain a sudden disaster, like a
landslide. Also notable how Hacquet describes the surface where the
landslide “slipped away”.

Fig.3. Detail of the modern geological map (Carta Geologica delle Tre Venezie, Foglio 12 "Pieve di Cadore", 1940) showing the village and the landslide of Alleghe (with the lake in the lower part of the map) and the dammed lake.

The landslide of Alleghe killed 48 people and destroyed parts of the village of Riete and some farms. The landslide-lake inundated the village of Peron, only in February 1771 a new outflow formed and stabilized the lake level.